The present invention relates generally to a heat exchanger and more particularly to a heat exchanger having a temperature sensor for monitoring the temperature of the coolant passing in contact with the heat transfer surfaces of the heat exchanger.
Many chemical reactions require close control of the reaction temperature. For example, in the pharmaceutical industry, it is not uncommon to use the batch method to prepare certain pharmaceuticals in a reactor vessel wherein the process temperature is as low as xe2x88x92150xc2x0 F. (xe2x88x92100xc2x0 C.) or even colder and down to temperatures as cold as xe2x88x92184xc2x0 F. (xe2x88x92120xc2x0 C.). Moreover, it is desirable to maintain the temperature of the reactor vessel within a narrow range and preferably at the lower end of the range so as to prolong the reaction time. Extending the batch reaction time provides a greater control over the reaction and an improvement in quality of the product. For example, if the acceptable range of the process temperature is (xe2x88x92100xc2x0 F. to xe2x88x92150xc2x0 F. xe2x88x9273xc2x0 C. to xe2x88x92100xc2x0 C.), each 18xc2x0 F. (10xc2x0 C.) decrease of the reaction temperature within this range can double the reaction time. Accordingly, a process temperature at the lower end of this range and as low as possible is preferred. Conversely, an increase of the temperature within the operating range speeds the reaction, decreases the quality of the materials produced and endangers the control over the reaction. There even is a danger of a runaway reaction should the temperature rise above the preferred range.
In order to maintain the proper operating temperature, the reactor vessel usually is jacketed. A low-temperature heat transfer fluid circulating through the jacket removes the exothermic heat of the reaction and heat gained from the surrounding environment. The heat transfer fluid in turn is circulated through an external heat exchanger in order to reject the heat gained during passage about the reactor vessel. Whereas a low-temperature heat transfer fluid is used for cooling the reactor vessel, a cryogen such as liquid nitrogen is used as the cooling fluid in the heat exchanger to remove heat from the low-temperature fluid heat transfer fluid.
Conventional practice is to measure the average or xe2x80x9cbulkxe2x80x9d temperature of the heat transfer fluid as it exits the heat exchanger. In response to this measure, adjustments are made by changing either the flow rate of the heat transfer fluid through the heat exchanger or by changing the flow rate of the cryogen. Relying upon the average outlet temperature of the heat transfer fluid (the xe2x80x9ccooled fluidxe2x80x9d) to initiate flow changes has not been entirely satisfactory. This is because the temperature that is measured is merely the average temperature of the cooled fluid leaving the external heat exchanger and is not necessarily a correct indication of the cooling condition at the interface between the cooled fluid and the cryogen.
It should be appreciated that the temperature of the heat transfer surface in contact with the cooled fluid is actually below the freezing point of the cooled fluid, especially where a cryogen such as liquid nitrogen is used as the medium to remove heat from the cooled fluid. Accordingly, it is important to the operation of the heat exchanger that conditions be maintained so as to avoid the freezing of the cooled fluid onto the heat transfer surface. This is because there is a marked difference in the temperature of the heat transfer surface and the operation of the heat exchanger when the cooled fluid freezes onto the heat transfer surface.
For example, once the cooled fluid begins to freeze onto the heat transfer surface, there is a dramatic temperature change, due in part to the insulation properties of the ice collecting on the heat transfer surface. The build up of ice on the heat exchange surfaces also may restrict flow passages through the heat exchanger. The restriction of the flow passages and the insulation provided by the build up of an ice layer act to compromise the thermal efficiency of the heat exchanger. However, the decrease in the heat exchange capability may not be immediately recognized because the average or bulk temperature of the cooled fluid at the outlet to the heat exchanger still may be within acceptable limits. So long as the temperature sensor sees that the cooled fluid leaving the heat exchanger is at an acceptable level, no corrective measures are taken. Accordingly, relying on the bulk temperature of the cooled fluid delays the taking of corrective action.
Each one of various factors plays a roll in determining whether the cooled fluid begins to freeze onto the heat transfer surface. Among these are the geometry of the heat exchanger and the physical properties of the cooled fluid. Other factors affecting the onset of freezing include the velocity of the cooled fluid across the heat transfer surface, the turbulence of the boundary layer at the heat transfer surface and the thermal diffusivity of the cooled fluid. Also, a heat transfer fluid made of a single compound such as methanol may freeze quickly once a certain temperature is reached. Other fluids comprising a mixture of different organic isomers having different freezing points may not be compromised as quickly since one or more components of the mixture may remain fluid even while other components may freeze. Accordingly, in view of the many variables that may have an affect on the onset of freezing, it is difficult to predict whether a given cooled fluid will freeze under a given set of conditions. Once freezing begins, the temperature of the heat transfer surface may very quickly experience a decrease of from 50xc2x0 to 100xc2x0 F. (28xc2x0 to 56xc2x0 C.) or more and this decrease promotes further rapid freezing.
As noted above, the conventional method of relying on the bulk temperature of the cooled fluid leaving the heat exchanger is not reliable because the bulk temperature can remain within acceptable limits for some time after the onset of freezing. At some time however, a condition termed xe2x80x9crunaway freeze-upxe2x80x9d may occur, which results in a dramatic loss of heat rejection capability by the heat exchanger. Corrective action after the cooled fluid temperature goes beyond an acceptable limit usually takes the form of decreasing the flow of the cryogen until the ice build up on the coils is removed. Unfortunately during this period the ability of the low temperature heat transfer fluid to remove heat of the reaction from the reactor is compromised. Should the reactor temperature increase beyond acceptable limits, the pharmaceutical produced either is of a poorer quality or must be discarded.
Accordingly, it is an object of the present invention to provide a heat exchange method and apparatus for more accurately monitoring the thermal condition of a fluid being cooled by the heat exchanger.
Another object of the present invention is to provide a heat exchange method and apparatus wherein the thermal condition of a fluid being cooled is measured independently of the fluid itself.
Yet another object of the present invention is to provide a heat exchange method and apparatus wherein the thermal condition of a heat exchange fluid is controlled by monitoring the surface temperature of a heat exchange surface.
A further object of the invention is to provide a heat exchange method and apparatus for controlling the temperature of heat exchange fluid used in cooling a pharmaceutical reactor.
In accordance with the method of the present invention, a low temperature heat transfer fluid is circulated about a pharmaceutical reactor to remove the exothermic heat of the reaction and maintain the reactor vessel at a substantially constant temperature. The heat transfer fluid in turn is circulated through a heat exchanger where the heat of the reaction picked up by the fluid is released to a cooling fluid. In this respect the heat transfer fluid is circulated through a heat exchange apparatus and into contact with cooling coils within the heat exchanger. A cryogen such as liquid nitrogen circulates through the coils to remove the heat gained by the heat transfer fluid.
The temperature of the cryogen circulating through the cooling coil may be well below the freeing temperature of the heat transfer fluid (the xe2x80x9ccooled fluidxe2x80x9d). However, during operation it is important to prevent the cooled fluid from freezing onto the surface of the cooling coils. Generally, the heat exchanger design and operating parameters are selected to prevent freezing. However, as noted above, it is difficult to predict whether a given heat transfer fluid will freeze under a given set of conditions and the bulk temperature of the cooled fluid leaving the heat exchanger is not an accurate indicator of freezing at any given location within the heat exchanger.
Accordingly, in accordance with the present invention, a temperature sensor is located so as to monitor the temperature of the heat exchange surface itself rather than the temperature of the cooled fluid leaving the heat exchanger. Should the sensor detect a temperature indicating the cooled fluid is freezing onto the heat transfer surface, a signal is sent to throttle the flow of the cooling fluid (such as a cryogen). Decreasing the flow of the cooling fluid for a time allows the heat transfer surface to warm and cause the melting of any cooled fluid frozen onto the surface. In this way there is no substantial reduction of the heat rejection capacity of the heat exchanger and the flow of the cooled fluid through the exchange is in no way diminished. This is because the ice melting releases its heat of fusion as it rejects the heat that was picked up by the heat transfer fluid. The heat of fusion, plus reduced of cooling fluid, still is sufficient to remove heat from the low temperature heat transfer fluid.
When the temperature sensors see that the temperature of the heat transfer surface has risen to a point above the temperature where the cooled fluid might freeze, the flow of the cooling fluid is restored. In this fashion there is a proactive control of the cooling fluid to prevent or remove the ice build up onto the heat exchange surface rather than a reactive control responding to the temperature of the cooled fluid leaving the heat exchanger. Accordingly, the present invention may be characterized in one aspect thereof by a method of operating a heat exchanger including a heat transfer wall having a first surface and an opposite second surface comprising:
a) passing a cooled fluid in contact with the first surface of the heat transfer wall and passing a cooling fluid in contact with the second surface of the heat transfer wall;
b) directly monitoring the temperature of the first surface of the heat transfer wall; and
c) adjusting the flow of one of the cooled fluid and cooling fluid in response to the temperature of the first surface to prevent the freezing of the cooled fluid onto the first surface of the heat transfer wall.
In another aspect, the present invention may be characterized by a heat exchange apparatus comprising:
a) a housing containing flow paths for a cooled fluid and a cooling fluid, the housing having a heat transfer wall with a first surface for contacting the cooled fluid and a second surface for contacting the cooling fluid;
b) means for directly monitoring the temperature of the first surface of the heat transfer wall; and
c) means acting responsive to the temperature of the first surface for adjusting the flow of one of the cooled fluid and cooling fluid through the heat exchanger so as to prevent the freezing of the cooled fluid onto the heat exchange surface.